1. Field of the Invention
This invention relates to drying of particulate carbonaceous material such as coal, lignite, and the like. More particularly, this invention relates to a process for removing water from particulate carbonaceous material by a method which reduces the amount of heat required to obtain a given degree of dryness.
Raw coal as it comes from the mines is frequently subjected to a washing operation, resulting in coal particles having a high degree of surface moisture. This moisture leads to difficulties in handling and shipping, and various methods of dewatering coal have been utilized over the years.
Many lignite materials contain a very high amount of moisture. In some cases, the amount of so-called "inherent" moisture in lignite particles is as high as 65 percent by weight. At least part of this moisture must be removed in order to obtain efficient burning of the lignite and also to reduce the cost of transporting the material.
2. Description of the Prior Art
Several methods for dewatering coal have been practiced over the years. These include methods of mechanical drainage as well as filtration. Other methods used separately or in combination with mechanical water removal include fluid bed drying and other conventional drying techniques. The "Convertol" process in which a coal slurry is mixed with heavy oil and then passed through a centrifugal dryer has been widely used. U.S. Pat. Nos. 2,176,902; 3,381,388; and 3,520,067 are representative of patents describing these prior art processes.
In the drying of materials other than carbonaceous material, it has been proposed to remove water by utilizing a low boiling solvent such as methanol. U.S. Pat. No. 1,687,588 describes a method of drying corrodible materials by repeatedly washing the moist material with alcohol and then evaporating the alcohol from the surface of the particles. U.S. Pat. No. 3,374,550 describes a process for drying a sheet of paper as it is formed by passing the moist sheet through methanol to replace part of the water, followed by drying the sheet of paper in a conventional manner. It is stated that the energy required to obtain the desired degree of dryness is reduced in this manner, or the drying can take place at a lower temperature.
Prior to this invention, there has been no teaching of a method of reducing the moisture content of coal or lignite utilizing a solvent having a low heat of vaporization and low boiling point.